The invention relates to shutter mechanisms of the type including an aperture-unblocking member mounted for movement between an unactivated aperture-blocking position and an activated aperture-unblocking position, and a drive mechanism coupled to the aperture-unblocking member. The type of shutter mechanism in question furthermore includes a braking mechanism driven by the drive mechanism when the aperture-unblocking member moves from the aperture-blocking position to the aperture-unblocking position. During such movement of the aperture-unblocking member, the braking mechanism driven by the drive mechanism applies to the latter braking force slowing the movement of the aperture-unblocking member to its aperture-unblocking position. The drive mechanism which drives the braking mechanism during movement of the aperture-unblocking member from its blocking to its unblocking position may either be a mechanism for actually driving such member to its unblocking position, or alternatively may be driven by the aperture-unblocking member during the movement of the latter, or may be driven by the drive mechanism for the aperture-unblocking member. Often, such shutter mechanism additionally include a distinct aperture-blocking member movable from an aperture-unblocking position to an aperture-blocking position, for terminating an exposure operation.
With shutter mechanisms employing such braking mechanisms to slow the movement of the aperture-unblocking member from the blocking to the unblocking position, problems are often encountered. In the first place, to slow the movement of the aperture-unblocking member in a proper manner, it would in principle be desirable to have the braking mechanism apply to the drive mechanism a braking force which is initially relatively low, so that high acceleration of the aperture-unblocking member at the start of its movement would not be prevented. On the other hand, once the speed of movement of the accelerating aperture-unblocking member has reached the design value, the braking action should become quite decisive in character. We have found that the prior art does not provide braking mechanisms capable of simply and reliably enough achieving this desirable manner of operation. Also, braking mechanisms of the prior art are very often characterized by high frictional wear which as it progresses may produce a progressive alteration in the braking action actually afforded, i.e., a progressive departure from the braking action contemplated by the designer.